6.5 Observations Outside Earth's Atmosphere

Overcoming Atmospheric Blockage: Earth’s atmosphere blocks most radiation at wavelengths shorter than visible light, including ultraviolet, X-ray, and gamma-ray radiation. Space-based observatories are essential to make direct observations in these bands.
Minimizing Atmospheric Distortion: The atmosphere distorts visible and infrared wavelengths, causing stars to “twinkle.” In space, these distorting effects are eliminated, allowing for clearer images and the observation of finer details limited only by the instrument’s size.
Enabling Optical System Cooling: In the vacuum of space, optical systems can be cooled to hundreds of degrees below freezing without condensing water vapor or other gases, which is crucial for sensitive infrared observations by nearly eliminating the instrument's own infrared radiation.

Challenge of Water Vapor: Water vapor in Earth’s lower atmosphere is the primary obstacle for infrared observations. Moving to higher elevations or into space significantly reduces this interference.
Early Airborne Efforts: Infrared observations from airplanes began in the 1960s. NASA operated a 0.9-meter airborne telescope from 1974 to 1995, flying at an altitude of 12 kilometers to get above $99\%$ of atmospheric water vapor.
Stratospheric Observatory for Infrared Astronomy (SOFIA): A more recent partnership between NASA and the German Aerospace Center, SOFIA, featured a 2.5-meter telescope on a modified Boeing 747SP, allowing observations above most atmospheric water vapor.
First Orbiting Infrared Observatory (IRAS): Launched in 1983 as a joint project by the U.S., Netherlands, and Britain, IRAS had a 0.6-meter telescope cooled to below $10 K$ . It conducted a comprehensive survey of the entire infrared sky, cataloging about $350,000$ sources.
Spitzer Space Telescope: Launched in 2003, this 0.85-meter infrared telescope operated until 2020, offering improved sensitivity and resolution. It has since been succeeded by the James Webb Space Telescope.
Importance of Space Infrared Telescopes: These telescopes are vital for detecting cooler cosmic objects, such as dust clouds around forming stars and remnants of dying stars, which are invisible in visible light.

Launch and Significance: Launched in April 1990, HST, with its 2.4-meter mirror, has become one of astronomy's most important telescopes. It was named after Edwin Hubble, who discovered the expansion of the universe.
Initial Flaw and Repair: After launch, a slight error of about $1/50$ the width of a human hair in its primary mirror caused blurry images. This was corrected during a challenging 1993 mission by Shuttle astronauts who installed compensating optics and a new camera.
Operational Aspect: Jointly operated by NASA’s Goddard Space Flight Center and the Space Telescope Science Institute, HST was designed to be serviced by astronauts, though this program has been discontinued.
Major Achievements: HST has provided incredibly detailed images, including the Hubble Ultra-Deep Field, an image of a small sky region observed for nearly 100 hours, revealing about $10,000$ galaxies, some formed when the universe was only a few percent of its current age.

Design and Launch: The largest space telescope to date, launched on December 25, 2021, JWST is specifically designed for observing infrared radiation. It is named after a former NASA administrator.
Location and Cooling: Orbiting about $1.5 \text{ million km}$ from Earth (four times further than the Moon), it is in a cold location ideal for infrared viewing. Its multi-layer sun shield, the size of a tennis court, protects its liquid-helium-cooled instruments.
Mirror and Resolution: JWST features an 18-segment primary mirror measuring 6.5 meters in diameter. Its resolution is so high it could distinguish a U.S. penny held up 40 km (24 miles) away.
Scientific Aims: JWST allows astronomers to examine dusty star-forming regions, study the atmospheres of exoplanets, and look back in time to when the earliest galaxies were assembled.
Infrared Imaging: The images produced by JWST are infrared and are assigned colors by science teams to highlight scientifically interesting details, as the different infrared wavelengths are not visible to the human eye.

Space-Exclusive Observations: Direct observations of ultraviolet, X-ray, and gamma-ray radiation can only be made from space because Earth’s atmosphere blocks these high-energy electromagnetic waves.
Early Development: Such observations began in 1946 using V2 rockets captured from Germany, initially for detecting solar ultraviolet radiation.
Chandra X-ray Observatory: Launched in 1999, Chandra produces X-ray images with unprecedented resolution and sensitivity, revealing the universe at short wavelengths.
Fermi Gamma-ray Space Telescope: Launched in 2008, Fermi is designed to measure cosmic gamma rays at energies higher than previous telescopes, collecting radiation from the most energetic events in the universe.
Technological Challenges: Designing instruments to collect and focus penetrating radiation like X-rays and gamma rays, which normally pass straight through matter, poses significant technological hurdles.
Ground-based Gamma-ray Detection: Indirect gamma-ray detections can be made from Earth by observing the cascade of light and energy produced when gamma rays hit the atmosphere and accelerate charged particles. Examples include the VERITAS array in Arizona and the H.E.S.S. array in Namibia.